The present invention relates generally to preprimed metal substrates, such as sheets, which are suitable for formed metal applications, for example, the manufacture of automotive body parts for repair and/or on-line assembly.
In the usual formed metal applications, a metal sheet is first formed by well-known methods (drawing, die cutting, etc.) into the desired shape then subsequently coated with a primer (postpriming). The primer serves a number of well-known functions, including providing a barrier layer and/or corrosion protection layer for the underlying metal substrate.
Typical examples of primers for metal applications, such as automotive applications, include the following.
U.S. Pat. No. 4,602,053 describes epoxyester linear block oligomers made with n-1 moles of fatty acid groups and n moles of epoxy oligomer, which are said to be particularly suited for chip-resistant coating compositions (preferably containing small pigment volume concentrations). See also EP-A-0179281.
WO 85/00375 discloses thermosetting coating compositions based upon an epoxy-polyester graft copolymer and a polyisocyanate crosslinker, which is said to be useful as a chip resistant primer for the automotive industry.
EP-A-0070008 describes zinc rich coatings having binders comprised of thermoplastic polyhydroxyethers, epoxy resins, epoxy ester resins, alkyl silicates, etc., which are useful for corrosion protection in the automotive industry. These coatings particularly contain aluminum trihydrate for improved combined spot welding/corrosion resistance properties on automotive components.
In automotive applications where the formed metals may take a number of odd or complex shapes, postpriming by normal on-line painting methods may result in a significant portion of the underlying metal substrate being unprimed or underprimed, as in the normal on-line painting methods it is difficult to adequately apply the primer in the various corners and other odd-shaped areas of the formed metal, particularly on both sides. In many instances, the various body parts are not primed until after assembly, thereby exacerbating the problem of unprimed or underprimed base metal. A common result is the early rusting and decreased life of the part.
It is, therefore, an object of the present invention to overcome this shortcoming by providing a preprimed metal substrate which may subsequently be formed into a shape suitable for use in automotive applications.
The general concept of preprimed metal substrates for formed metal applications is, of course, not new. Most primers suitable for use in automotive applications, however, are not suitable for preprimed formed metal applications, for example, because of either insufficient adhesion to the metal substrate or insufficient flexibility. In forming the preprimed metal substrate, particularly in the manner and degree necessary for automotive applications, insufficient adhesion can cause the primer to separate from the substrate, while insufficient flexibility can cause the primer to crack. Both can result, as indicated above, in the early rusting and decreased life of the part.